More restrictive exhaust emissions laws for diesel engines have driven changes in engine design including the need for high-pressure electronic fuel injection systems. Engines built according to the new designs use higher combustion pressures, higher operating temperatures and less lubrication than previous designs. Components of the new designs, including valve seat inserts (VSI), have experienced significantly higher wear rates. Exhaust valve seat inserts and valves, for example, must be able to withstand a high number of valve impact events and combustion events with minimal wear (e.g., abrasive, adhesive and corrosive wear). This has motivated a shift in materials selection toward materials that offer improved wear resistance relative to the valve seat insert materials that have traditionally been used by the diesel industry.
Another emerging trend in diesel engine development is the use of EGR (exhaust gas recirculation). With EGR, exhaust gas is routed back into the intake air stream to reduce nitric oxide (NOx) content in exhaust emissions. The use of EGR in diesel engines can raise the operating temperatures of valve seat inserts. Accordingly, there is a need for lower cost exhaust valve seat inserts having good hot hardness for use in diesel engines using EGR.
Also, because exhaust gas contains compounds of nitrogen, sulfur, chlorine, and other elements that potentially can form acids, the need for improved corrosion resistance for alloys used in exhaust valve seat insert applications is increased for diesel engines using EGR. Acid can attack valve seat inserts and valves leading to premature engine failure. Earlier attempts to achieve improved corrosion resistance were pursued through the use of martensitic stainless steels. Though these steels provide good corrosion resistance, conventional martensitic stainless steels do not have adequate wear resistance and hot hardness to meet the requirements for valve seat inserts in modern diesel engines.
Cobalt-based valve seat insert alloys are known for their high temperature wear resistance and compressive strength. A major disadvantage of cobalt-based alloys, however, is their relatively high cost. Iron-based VSI materials, on the other hand, typically exhibit a degradation in matrix strength and hardness with increasing temperature, which can result in accelerated wear and/or deformation. Iron-based alloys for use in internal combustion engine valve seats are disclosed in U.S. Pat. Nos. 6,702,905; 6,436,338; 5,674,449; 4,035,159 and 2,064,155.
There is a need in the art for improved iron-based alloys for valve seat inserts that exhibit adequate hot hardness, high temperature strength and low cost, as well as corrosion and wear resistance suitable for use in exhaust valve seat insert applications in diesel engines using EGR.